The present invention relates in general to a system for controlling the application of a solution to printing press rollers, and more particularly, to improved methods and apparatus for applying a wetting solution in the form of a pulsating spray, and controlling the amount of solution sprayed in accordance with the speed of the printing press.
By the nature of the planographic printing processes, including offset and photolithography, an application of a wetting solution and ink are applied to inking rollers and the ink is then transferred to the paper medium. The wetting solution is applied to the roller including the portion representing the absence of printed information. The wetting solution is basically water along with other additives. According to current offset printing methods, the ink is of an oil base composition, and a viscosity which repels the water base solution. In conventional planographic printing techniques the ink adheres to the photoengraved portion of the plate, and is repelled by the blank portion. Also, the wetting solution adheres to the blank portion, or nonphotoengraved part of the plate, and thus further prevents the application of ink, especially excess ink, on the blank areas of the planographic plate.
The transfer of the ink and wetting solution to the rotating photoengraved plate is accomplished by causing such plate to contact a rotating inking roller which has deposited or sprayed on it a first coating of ink, followed by a second outer coating consisting of the wetting solution.
Control of the amount of wetting solution on the inking roller is crucial for several reasons. If insufficient wetting solution is applied to the roller the ink tends to migrate onto the blank portion of the plate and thereby be transferred to corresponding areas of the paper which are not to be printed. Secondly, if an excess amount of wetting solution is applied to the roller, such excess represents waste which must be collected and removed from the system.
The problem of accurately applying a wetting solution in planographic printing processes has been addressed before, and has to a certain degree succeeded. However, with the speed and efficiency at which printing systems currently operate, the shortcoming of these traditional techniques cannot be tolerated.
One type of spray system described in U.S. Pat. No. 3,937,175 by Horner employs a solenoid controlled valve in the fluid supply line to intermittently supply fluid in a pulsed manner to spray nozzles. The solenoid is electrically controlled by a timer to open the valve when the system pressure is at a maximum to thereby achieve a forceful jet of spray. This system suffers a shortcoming insofar as it does not take into account the speed of the press via-a-vis the amount of spray discharged.
A more sophisticated spray dampening system is disclosed in U.S. Pat. No. 4,064,801, and assigned to the assignee of the present application. In this system the pressure of the dampening fluid is regulated in accordance with the speed of the press whereby a metered spray can be applied to the press rollers. Means are also provided for pulsing the spray nozzles open and closed, as well as for manual adjustment of the duty cycle and frequency of such pulses. The difficulty prevalent in these systems is that despite an attempt to very accurately gauge the spray pressure with press speed, the resultant spray is linearly related to press speed and thus at low speeds too little liquid is applied, and at high speeds too much liquid is applied.
More particularly, the spray dampening system disclosed in Pat. No. 4,064,801 is an air system, that is, the air carries the dampening fluid to the press rolls. The air is under a pressure of about 2 psi and the pressure of the liquid varies from 1 to 5 psi depending upon the press speed. Such a low pressure system has more tendency to clog than a high pressure airless system in which the nozzles are in effect "self cleaning".
In an airless sprayer, however, the pressure of the liquid governs the spray pattern angle and must remain constant. If it does not, the spray pattern angle will change causing variations in the coverage on the press roll.
It is therefore an object of this invention to provide an airless sprayer where the liquid pressure and thus the spray pattern angle or coverage of the rolls remains constant and there is greater control over, and increased uniformity in the application of the liquid.
It is a further object to provide an instantaneous shut off of liquid during each pulse cycle without perceptible variation in the spray pattern angle or coverage of the rolls.
It is also an object of the present invention to provide a pulsed spray dampening system where a liquid is supplied to spray nozzles at a constant pressure, but where such nozzles are automatically pulsed at frequencies corresponding to various press speeds.
It is another object of the present invention to provide a pulsed spray dampening system where the amount of spray liquid is nonlinearly related to the speed of the press such that at low and high press speeds the amount of liquid is respectively increased and decreased so as to optimize the process and minimize waste.
It is a corollary object of the present invention to provide a pulsed spray dampening system which automatically adjusts, according to a predetermined scheme, the liquid spray volume by adjusting the frequency by which the liquid is pulsed by the nozzles. It is yet another object of the invention to section the spray dampening system into major functional units and isolate each unit electrically to prevent electrical failures in one such unit from affecting another unit thereby enhancing maintenance, repairability and reliability of the system.